How to copy weight painting on foot1 to foot2?
It seems to me that the new foot bone is either using the group associated with the other foot or that it copied a new group to the new foot but has also copied over the weights for the opposite foot...

You need to make sure that you have a separate group for each foot bone, and that each of those groups is associated with its particular foot weights.

Its also good to keep good naming conventions, foot.R and foot.L, for your bones and groups as blender uses the .L or .R to work out its symmetry when doing opposite calculations.

Hope this helps...